In the recovery of oil from a subterranean oil-bearing formation only between about ten to fifty percent of the oil in place is recoverable using combined primary and secondary production modes. As a result, tertiary or enhanced oil recovery processes have been developed. Such processes include thermal processes exemplary of which are steam flooding and in-situ combustion, chemical flooding techniques and gaseous displacement drives. The gases used may include steam, carbon dioxide or hydrocarbons.
However, several problems occur when a gas phase is used as the displacing medium. First, fingering of the gas phase into the oil will degrade the uniform displacement front with concomitant reduction in oil recovery. This is a result of the adverse mobility ratio between the displacing gas and the oil. Secondly, the density difference between the gas and oil phase will cause gravity override wherein the gas will tend to move upwardly, sweeping only the upper portion of the oil-bearing zone. Finally, reservoir heterogeneities and zones of relatively well swept (i.e. low oil) rock can cause the displacement fluid to channel through the oil-bearing zones. All of these phenomena act to reduce the amount of oil recovered.
The use of surfactant-stabilized foams comprises a relatively new technology for circumventing these problems.
The foam, having a viscosity greater than the displacing medium, will preferentially accumulate in the well-swept and/or higher permeability zones of the formation. The displacing medium is thus forced to move into the unswept or underswept areas of the formation. It is from these latter areas that the additional oil is recovered. However, when a foam is used to fill a low oil content area of the reservoir, the oil contained therein is, for all practical purposes, lost. This is because the foam functions to divert the displacement fluid from such areas.
The selection of suitable foam-forming surfactants which produce foams having the necessary stability to collapse and viscosity is crucial. Such properties as solubility, surface tension and bulk foam stability must be taken into consideration. Typical tests for the evaluation of surfactants would include solubility tests in salinity, and temperature environment of the particular reservoir, bulk foam tests to ensure the stability of the foam to collapse, and permeability or pressure drop measurements made in packed sand beds or cores containing injected foam. U.S. Pat. Nos. 4,589,276 to Djabbarah and 4,601,336 to Dilgren et. al. cover some of these tests.
Recent studies have indicated that many foams ar destroyed upon being brought into contact with an oil phase. It is desirable that the foam not collapse upon contact with the residual oil in the swept portions of the reservoir, nor block unswept portions thereof.
The classical description of foam-oil interactions has been outlined by S. Ross, J. Phys. Colloid Chem. 54(3) 429-436 (1950). It has been held that foam stability in the presence of oil can be described from thermodynamics in terms of the Spreading and Entering Coefficients S and E respectively. These coefficients are defined as follows: EQU S=Y.sup..degree..sub.F -Y.sub.OF -Y.sup..degree..sub.O
wherein
Y.sup..degree..sub.F is the foaming solution surface tension; PA1 Y.sub.OF is the foaming solution-oil interfacial tension; and PA1 Y.sup..degree..sub.O is the surface tension of the oil. EQU E=Y.sup..degree..sub.F +Y.sub.OF -Y.sup..degree..sub.O PA1 Y.sup..degree..sub.F, Y.sub.OF and Y.sup..degree..sub.O are as defined supra. PA1 R.sub.O is the radius of an emulsified drop; PA1 Y.sub.OF is the foaming solution-oil interfacial tension; and PA1 r.sub.p is the radius of a foam lamella Plateau border where it initially contacts the oil. The Plateau border refers to the part of a foam lamella that has curved surfaces. Plateau borders occur where a foam lamella meets either another foam lamella, or a surface of another material such as oil or solid.
wherein
Based on these coefficients, one can predict that three types of oil-foam interactions could take place. First, (Type A) an oil will neither spread over nor enter the surface of foam lamellae when E and S are less than zero. Secondly, (Type B) oil will enter but not spread over the surface of foam lamellae when E is greater than zero but S is less than zero. Thirdly, (Type C) oil will enter the surface of foam lamellae and then spread over the lamellae surfaces if both E and S are greater than zero. This latter behaviour, typically, will destabilize the foam. However, experimental results have not borne out these predictions. Furthermore, the theory was developed assuming that the oil droplets are readily imbibed into the foam lamellae. Again however, experimental results show that some foams, particularly those of type A supra do not readily imbibe oil.
There exists, therefore, a need to distinguish between foams which are stable to oil but do not significantly imbibe oil, as in type A supra, foams which are stable to oil and do imbibe oil as in the second type above and finally, foams that are unstable to oil as in the third predicted type.